Regulated or controlled power supplies are found in virtually all electronic devices, including battery chargers, cellular telephones, computers, computer monitors, televisions, audio equipment and video cameras. One typical power supply is a converter, such as a direct current to direct current converter (in the following simply designated as DC converter), which operates from a power source, generates an alternating signal as an intermediate process and delivers an output signal to a load. The DC converter accepts a DC input voltage and produces a DC output voltage. Typically, the produced output voltage is at a different value or level than the input voltage.
In a typical pulse width modulation (PWM) regulator circuit, a square wave is provided to the control terminal of a switching device to control its on-states and off-states. Since increasing the on-time of the switching device leads to an increase of the output voltage and vice versa, the output voltage may be controlled by manipulating the duty cycle of the square wave. This manipulation is accomplished by a control circuit in a control loop, which continuously compares the output voltage with a reference voltage and adjusts the duty cycle of the square wave to maintain a substantially constant output voltage.
As an alternative, pulse frequency modulation (PFM) of the voltage regulation provides better efficiency at small output current levels than the above PWM mode. First, a PFM mode requires fewer turn-on transitions to maintain a constant output voltage than a PWM mode, resulting in a lower gate-drive power dissipation of the switching transistor. Second, since the PFM mode can be achieved with a much simpler control circuit having fewer components, the power dissipation in a control loop of the PFM mode is less than in the control loop of the PWM mode. However, when the output current reaches a moderate level, the PFM mode of voltage regulation becomes impractical, since the maximum output current available from the PFM mode is generally much less than that available from the PWM mode.
FIG. 1 shows a schematic block diagram of a conventional converter circuit which generates a regulated output voltage Vout from a variable input voltage Vin. The output voltage Vout can have a higher value than the input voltage Vin and is substantially constant, although the input voltage Vin and the output load may change. Such DC voltage converters usually use an inductor L to store energy generated by a current flowing through the inductor L and a switching device 20, which may be a power transistor or another controllable semiconductor switching device. The switching device 20 is used to switch off the respective current path, so that the energy stored in the inductor L is then transmitted as a current via a diode D to the output and charges a capacitor C connected in parallel to the output terminal. By continuously switching on and off the switching device 20, the energy stored in the inductor L is continuously transferred via the diode D to the capacitor C and charges the capacitor C. The diode D serves to provide decoupling between the voltage at the capacitor C and the voltage at the switching device 20, so that the output voltage Vout can be higher than the input voltage Vin. As already mentioned, the switching device 20 may be controlled in a PWM operating mode with a fixed frequency, wherein the duty cycle or the duration of the switching phase is controlled in order to keep the output voltage Vout substantially constant.
On the other hand, the switching device 20 may be operated in a PFM operating mode, wherein the switching frequency is changed in order to keep the output voltage Vout substantially constant. The switched operating mode is controlled by a driver circuit 10 comprising an oscillator and generating a corresponding control signal, such as a rectangular signal, supplied to the control terminal of the switching device 20.
The output voltage Vout is regulated or controlled by a feedback loop 40 which compares the value of the output voltage Vout with a reference voltage and then adjusts the switching frequency or duty cycle in accordance with the comparison result. In order to improve efficiency of the converter, an additional switching device 30 may be provided at the diode D or instead of the diode D, in order to remove the threshold voltage of the diode D. The additional switching device 30 may be controlled by a separate driver device or by the driver device 10 that controls the switching device 20.
In the following, a complete three-phase operating cycle of the DC converter is described:
In a first phase, the switching device 20 is switched on and the additional switching device 30 is switched off, so that a current flows through the inductor L and the switching device 20 and energy is stored in the inductor L for one oscillator cycle.
In the second phase, the switching device 20 is switched off and the additional switching device 30 is switched on, so that the current now flows to the capacitor C and energy is transmitted to the capacitor C.
In the third phase, the switching device 20 and also the additional switching device 30 are switched off, e.g. between the first and second phases or when the output voltage Vout has reached the correct or desired voltage value.
The output voltage Vout is controlled by the feedback loop 40 which allows or initiates the start of a new operating cycle if the output voltage is too low, to thereby increase the switching frequency or the duty cycle. The switching phases must be carefully controlled by the driver device 10 in order to avoid the switching device 20 and the additional switching device 30 from being switched on simultaneously.
The amount of energy that can be transmitted to the output is directly linked to the inductance value of the inductor L and the switching period of the oscillator in the driver device 10. For a given inductance value and oscillator frequency, the desired output power can be delivered only for a limited input voltage range. As a consequence, unexpected or sudden drops of the input voltage beyond this limited range may lead to considerable changes in the output voltage.
Document US 2003/0214276 A1 discloses a DC converter, wherein a regulated output voltage and an output current are generated by using a switching device for providing the output current and controlling the switching device with a first control circuit functioning in a PWM mode and, in an alternative manner, with a second control circuit functioning in a PFM mode. A second feedback circuit is included in the second control circuit and a time delay is introduced in a second feedback circuit in order to introduce a limitation of pulse frequency. Thereby, it is possible to detect overload conditions, that may occur when the converter operates in the PFM mode, and to switch back to the PWM mode.